Method of removal of excess halides from photographic developing baths



March 16, 1937. J BLANEY I 2,973,621

METHOD CF REMOVAL OF EXCESS HALIDES FROM PHOTOGRAPHIC DEVELOPING BATHS Filed Oct. 6, 1933 I; u 3 1.2 o

I B v Log-Exposure FIG. 1.

a 4 z T I M H Developin Anode B I 136+! l-jlecoh'olyi'e 1C! 1 I 1 I I l e- Na BY----- 1 11 NaOH 4 Na CI- INVENTOR Jesse M Blanev ATTORNEY 25 halide.

Patented Mar. 16, 1937 UNITED STATES PATENT OFFICE METHOD OF REMOVAL OF EXCESS HAL- IDES FROM PHOTOGRAPHIC DEVELOPING BATHS This invention relates generally to photography and specifically to the reconditioning of photographic developing baths or solutions.

When an exposed photographic material is developed, three marked changes occur in the composition of the chemicals in the developer, namely:

} (a) the photo reducing agents are oxidized; (b)

' the equivalent of alkali is converted to a salt, thenature of which depends upon the type of developer; (c) the equivalent of alkali bromide is 7 formed.

\ In commercial work, and particularly in the motion picture industry, where processing on continuous machines is generally practiced, it is a necessity to time the print and to so maintain the developer that identical prints will be obtained without change in time of development.

In respect to the loss of photo=reducers and the depression of the pH (i. e.) increase in the hydrogen ion concentration in the developer, it is easy to maintain the bath by means of a suitable replenisher. But there has been no means for the removal of the constantly increasing photographic restraint of the accumulating alkali The current practice is to combat the restraint due to excess bromide, by increasing the content of alkali and photo reducers; but this does not correct the density contrast and it does affect 30 the grain and the color adversely.

Therefore, it is an object of the invention to provide a method of removal of excess halides from a photographic developer and of maintaining a normal hydrogen ion concentration by elec- 35 trolysis.

Other objects will appear and be better understood as the speciflcation proceeds, reference being made to the accompanying drawing forming a part thereof, and in which:

Figure 1 is a graph illustrating the effect of alkali halide in a developer.

Figure 2 is an elevation, partly in section, of

an'electrolytic cell, used in the electrolysis of the photographic developer, and demonstrating the 45 principles involved in elimination of the excess halides.

Referring more specifically to the drawing and especially to Figure 1, wherein the logomet-- rical exposures are plotted against logometrical opacities, it will be seen that the curve A represents a motion picture positive stock developed to a gamma of 2.0 in the bath recommended by the film manufacturers.

The curve B is a plot of the densities resulting 55 from the development for the same time as A in the same developer, but which has been worked until one half of the reducing agents has been oxidized. This developer has a content of 5.7

ms. per liter of sodium bromide, over the normal- 60 content.

Curve C illustrates the characteristics of development in bath B, to which reagents have been added so that the content of active chemicals is identical with that of bath A" except that the bromide content is still 5.7 gms. per liter in excess. Development time is the same as in A and B. Not only is the gamma sub-normal, but the densities deviate widely from those produced by A bath in the same time. 1

Curve D is plotted from a sensitometer exposure developed in bath C after photoreducers and alkali have been added to bath C in excess of the content in bath A. A gamma equal to A is offset by sub-normal densities.

Curve E is the result of further addition of chemicals to bath D to the end that one density alone is the same as in A, but all others are faulty while the gamma is abnormal.

It is, therefore, apparent that replenishment of the photo reducers is a necessity and small general additions of active reagents are required to compensate for mechanical and other losses, but

' numeral l represents a container for the developer and 2 a supplemental container or septum, composed of porous material, for containing an electrolyte preferably of an alkali halide such as sodium chloride and deposited in the container I.

An anode 3 is immersed in supplemental container 2, and a cathode 4 into container I and are suitably connected to a source of direct electric current, not shown in the drawing.

The chemical symbols show the migration of the alkali ion Na from the developer in container l and also from the electrolyte in supplemental container 2 to the cathode 4. The symbols also show the migration of the bromine ion Br from the developer through the porous container 2 to the anode 3. There is also indicated the reaction of the Na ion to alkali hydroxide NaOH with liberation of hydrogen H2 at the cathode 4 and chlorine C12 and bromine Brz at the anode 3. Both gases are liberated into the atmosphere.

Other electrolytes may be used such as spent developer, but sodium chloride is preferred as oxygen is not liberated from the anode and the cost is at the minimum.

In operation, a direct current is connected to the cell with the positive terminal connected to the anode 3, and the negative to the cathode 4. The potential varies with the type of cell, conductivity of the developer and other obvious factors, but 3.5 to 5 volts is representative. The minimum potential of decomposition of sodium bromide would require a rather large cell and it, therefore, is preferable to run the apparatus at about 60 to 70% efiiciency in respect to the elimination of the bromine from the developer.

The decomposition potential of sodium bromide, being in excess of that of alkali iodides, assures the 'removal of the iodine ion if present. On the other hand if the effective potential at the electrodes exceeds that. required for sodium bromide by about 0.2 volt, all three halogen ions will migrate into the anode electrolyte and will be liberated as gases after the anode electrolyte is supersaturated. The halogens are, therefore, not only removedfrom the developer, but are liberated from the anolyte as gases which are conducted away by suitable apparatus.

In operation, the sodium ion in the developer migrates to the cathodes and reacts with water with the production of sodium hydroxide and. the liberation of hydrogen. The latter is easily separated from the developer and is quite inert chemically on the developer at the pH of a practical solution.

When halogens are liberated from a photographic emulsion. during development, bromine for example oxidizes quinol to quinone with the production of acid hydrobromic which reacts with the sodium carbonate to sodium bromide and sodium bicarbonate. Since acid hydrobromic reacts to twice its equivalent in sodium bicarbonate, the latter buffers the developer strongly and cuts the reduction potential to a marked degree. This bufier is removed by the sodium hydroxide produced at the cathode and, if it were practical to run thecell at 100% efilciency, the pH of the electrolyzed developer would exceed the original hydrogen ion concentration but little. But in practice an overvoltage is employed to speed the migration of the ions and an excess of sodium hydroxide is manufactured at the cathodes; this excess is neutralized by periodic or continuous injection of dilute acid, preferably acid-sulfuric into the developer during electrolysis.

The sodium hydroxide formed in the developer in the immediate vicinity of the cathodes would ordinarily cause a great increase in pH locally and the consequent reaction to phenates with the photographic reducers. As phenates have an abnormal photographic reduction potential and reach equilibrium with buflered carbonates rather slowly, provision should be made to secure a strong circulation of the developer in the immediate vicinity of the cathodes, or the cathodes. themselves may be rotated by some means such as the pulley arrangement shown at 5.

I claim:

1. The method of removing excess bromide from photographic developer solutions which comprises electrolyzing the solution in a diaphragm cell, the developer solution being in the cathode compartment of the cell, whereby bromide is transferred through the diaphragm from the cathode compartment to the anode compartment.

2. The method of regenerating photographic developing solutions which have become oxidized through use, which comprises electrolyzing the oxidized solution in a diaphragm cell, the developing solution being in the cathode compartment of the cell.

3. The method of electrolytically decreasing the hydrogen ion concentration of a used photographic developing solution which comprises electrolyzing said solution in a diaphragm electrolytic cell, with the developing solution being in the cathode compartment of the cell.

4. The method of reconditioning a used photographic developing solution which comprises electrolyzing said solution in the cathode compartment of an electrolytic cell having two electrolytes, one of which is the developing solution and a catholyte, and the other an anolyte, which are separated by a porous partition permeable to halogen ions but resistant to diiiusion of the electrolytes, whereby excess halides are removed.

5. The method 01 reconditioning a used photographic developing solution which comprises electrolyzing said solution in the cathode compartment of an electrolytic cell having two electrolytes, one of which is the developing solution and a catholyte, and the other an anolyte, which are separated by a porous partition permeable to halogen ions but resistant to diffusion of the electrolytes, whereby excess halides are removed, and causing relative movement, during electrolysis, between the cathode and the electrolyte in contact therewith to prevent abnormal increase in the hydrogen ion concentration therein.

6. The method of reconditioning a used photographic developing solution which comprises electrolyzing the solution in a diaphragm cell with the solution being in the cathode compartment thereof, and introducing aphoto-reducer into the solution during electrolysis.

7. The method 01' reconditioning a used. photographic developing solution by reduction oi. excess halide content to a desired value, which comprises electrolyzing the used developing solution in a diaphragm electrolytic cell with the developing solution being in the cathode compartment thereof, the solution in the anode compartment containing an alkali halide, whereby the halogen ions migrate through the diaphragm into the anode compartment.

8. The method of reconditioning a used photographic developing solution and decreasing the hydrogen ion concentration to a desired value, which comprises electrolyzing said solution in a diaphragm electrolytic cell, with the developing solution being in the cathode compartment thereof, the solution in the anode compartment containing an alkali halide whereby the hydrogen and alkali ions migrate through the diaphragm into the cathode compartment of the cell.

9. The method of reconditioning a used photographic developing solution and simultaneously eliminating the halogen component of the excess halide and decreasing the hydrogen ion concentration, which comprises electrolyzing the used solution in a diaphragm electrolytic cell, with the developing solution being in the cathode compartment thereof, and introducing a reagent of low hydrogen ion concentration into said solution during electrolysis to neutralize excess hydroxide formed.

JESSE M. BLANEY. 

